1. Field of the Invention
The present invention relates to a projection system in which imaging light emitted from a projector is projected on a projection screen to produce thereon an image. More particularly, the present invention relates to a projection screen capable of sharply displaying an image and of providing high image visibility, and to a projection system comprising such a projection screen.
2. Background Art
Conventional projection systems for business and home uses are as follows: imaging light such as an image emitted from a projector is projected on a projection screen, and viewers observe the light reflected from the projection screen as an image. In such conventional projection systems, a projection screen comprising a transparent medium in which transparent or semitransparent, porous, finely divided particles are dispersed and a reflecting member provided at the rear of this transparent medium is used, for example.
However, it is difficult for such a conventional projection screen to show good image display performance under bright environmental light because it reflects not only imaging light but also environmental light such as sunlight and light from lighting fixtures.
In a conventional projection system, differences in the intensity of light (imaging light) projected on a projection screen from a projector cause light and shade to form an image. For example, in the case where a white image on a black background is projected, the projected-light-striking part of the projection screen becomes white and the other part becomes black; thus, differences in brightness between white and black cause light and shade to form the desired image. In this case, in order to attain excellent image display, it is necessary to make the contrast between the white- and black-indication parts greater by making the white-indication part lighter and the black-indication part darker.
However, since the above-described conventional projection screen reflects both imaging light and environmental light such as sunlight and light from lighting fixtures without distinction, both the white- and black-indication parts get lighter, and differences in brightness between white and black become small. For this reason, it has not been easy for the conventional projection screen to provide good image display unless the influence of environmental light, such as sunlight and light from lighting fixtures, on the projection screen is suppressed by using a means for shading a room, or by placing the projection screen in a dark environment.
Under these circumstances, studies have been made on projection screens capable of showing good image display performance even under bright environmental light. There have so far been proposed projection screens using, for example, holograms or polarized-light-separating layers (see Japanese Laid-Open Patent Publications No. 107660/1993 (Patent Document 1) and No. 540445/2002 (Patent Document 2)).
Of these conventional projection screens, those ones using holograms have the advantage that the white-indication part can be made lighter if their light-scattering effect is properly controlled, so that they can show relatively good image display performance even under bright environment light. However, holograms have wavelength selectivity but no polarization selectivity, so that the projection screens using holograms can display images only with limited sharpness. Moreover, because of production problems, it is difficult to produce large-sized projection screens by making use of holograms.
On the other hand, on the above-described conventional projection screens using polarized-light-separating layers, it is possible to make the white-indication part lighter and the black-indication part darker. Therefore, these projection screens can sharply display images even under bright environmental light as compared with the projection screens using holograms.
Specifically, Patent Document 1 describes a projection screen for which a cholesteric liquid crystal that reflects red, green and blue light (right- or left-handed circularly polarized light) contained in imaging light is used in order to make the projection screen not reflect nearly half the environmental light incident on the screen, by making use of the circularly-polarized-light-separating property of the cholesteric liquid crystal.
However, in the projection screen described in Patent Document 1, since the cholesteric liquid crystal is in the state of planar orientation, specular reflection occurs when the cholesteric liquid crystal reflects light, which makes it difficult to recognize the reflected light as an image. Namely, to recognize the reflected light as an image, it is necessary that the reflected light be scattered. However, Patent Document 1 is quite silent on this point.
On the other hand, Patent Document 2 describes a projection screen using, as a reflective polarization element, a multi-layered reflective polarizer or the like, having diffusing properties. This projection screen does not reflect part of the environmental light incident on it because of the polarized-light-separating property of the multi-layered reflective polarizer or the like, and scatters the reflected light by interfacial reflection that occurs at an interface between materials having different refractive indices, constituting the multi-layered reflective polarizer, or by means of a diffusing element that is provided separately from the multi-layered reflective polarizer. Further, Patent Document 2 describes a projection screen using a cholesteric, reflective polarizer or the like as a reflective polarization element in combination with a diffusing element. This projection screen does not reflect part of the environmental light incident on it because of the polarized-light-separating property of the cholesteric, reflective polarizer or the like, and scatters the reflected light by means of the diffusing element that is provided separately from the cholesteric, reflective polarizer.
Namely, the projection screen described in Patent Document 2 is made to selectively diffuse-reflect only a specific polarized component of the imaging light projected from a projector by making use of the so-called polarized-light-separating property so that the projection screen sharply displays an image, and is also made to scatter the reflected imaging light so that the projection screen provides improved image visibility.
However, in the projection screens described in Patent Documents 1 and 2, reduction of light reflection efficiency that occurs on the polarized-light separating layers for use in these projection screens is neglected. Namely, the polarized-light separating layer for use in such a projection screen is often a laminate of a plurality of selective reflection layers having different reflection wave ranges that cover the wave ranges for red (R), green (G) and blue (B) colors, the three primary colors of light. If the polarized-light separating layer is such a laminate, the state of polarization of light that has entered the polarized-light separating layer from the observation side is disturbed while the light is traveling toward the opposite side. Therefore, the light reflection efficiency of the selective reflection layer situated apart from the observation side becomes lower than that of the selective reflection layer situated closer to the observation side.
In order that the selective reflection layers constituting the above polarized-light separating layer can have the same light reflection efficiency, it is necessary to make the selective reflection layers different in thickness so that the selective reflection layer situated more distant from the observation side has a greater thickness. In this case, a greater increment in thickness is needed for the selective reflection layer whose reflection wave range covers the longer wavelength side. Therefore, for example, if the selective reflection layer having a reflection wave range covering the longer wavelength side is positioned apart from the observation side, this layer inevitably has an excessively great thickness. As a result, the polarized-light separating layer, as a whole, is to have a thickness greater than needed, and the function of selectively reflecting light possessed by the polarized-light separating layer is impaired.